It is known that film-forming polymers capable of orientation, such as isotactic polypropylene, can be transformed into manufactured items, such as sheets, tubes and shaped bodies in general, by thermoforming operations, such as by pressing, calendaring, extrusion, injection-pressing, vacuum forming, and the like.
The manufactured articles obtained by means of the above said procedures have mechanical and physical properties lower than those of the manufactured articles obtained from the same polymers and having a higly oriented structure, with consequent limitations in use.
To date, manufactured articles having a higly oriented structure, in the form of fibers and films, have been obtained by industrial processes.
In patent literature, high-thickness oriented manufactured articles are disclosed, by means of high-temperature compression of a set of oriented films.
According to British Specification No. 1316640, cross laminates of oriented olefin polymer films are obtained by extruding onto one of them a bonding film, and then laminating the assembly by applying pressure while the bonding film is molten and the oriented films are solid.
In this case, the bonding film can be a substantially unoriented stretchable polymeric material formed of the same monomers as those of the oriented films, but is different therefrom in that it has a lower melting point than the polymer of these latter films.
By such a method, composite products are obtained having an unsatisfactory resistance to delamination.
French patent No. 1478354 discloses the method for manufacturing laminated structures, which consists in subjecting to heat and pressure a composite formed of undirectionally oriented olefin polymer films, in the absence of any adhesive, at a temperature below the crystalline melting point.
A similar process, with the use of biaxially stretched films, is also described in French patent application No. 2055709.
By such methods, however, only films having a low degree of orientation of the crystalline phase, such as to give rise to X-ray scattering angles x higher than 10.degree., can be used with the result that composite structures characterized by low orientation degree, and consequently endowed with low modulus and other low tensile characteristics are obtained.
As a matter of fact, laminated structures formed of polypropylene films showing at the same time X-ray scattering angles not higher than 10.degree., and satisfactory resistence to delamination cannot be obtained from uniaxially oriented polypropylene films endowed with said scattering angles, by merely heating under pressure an assembly of same at a temperature lower than the crystalline melting point. In that case, only the use of bonding agents might ensure a sufficient adhesion between the films.
However the methods till now used involving the use of polymeric bonding agents have given rise to products endowed with not quite satisfactory resistence to delamination of the composite.